Multi-leaf collimators (MLC) are used (principally) in the field of radiotherapy. A beam of radiation is directed toward a patient and must be collimated to fit the shape of the area to be treated. It is important to ensure that the dose in the areas outside that shape is as low as possible, but also that the whole area is treated. If areas are left untreated then the likelihood of recurrence is increased, whereas if non-treatment regions are irradiated then damage will be caused to healthy tissue resulting in greater side effects and longer recovery times after treatment.
As the treatment area is rarely rectilinear, multi-leaf collimators are employed. These comprise an array of finger-shaped leaves of a radiation-absorbing material, each disposed in a parallel relationship and each able to move longitudinally relative to the others. By moving each leaf to a selected position, a collimator is provided which can exhibit a non-linear edge. In general, one such array (or “bank”) will be provided on each side of the beam.
Multi-leaf collimators generally suffer from two difficulties. One is the leakage of radiation between the leaves of the bank, and the other is that the leaves often have square ends and thus exhibit a pixellated pattern when aligned to an edge that is non-perpendicular to the leaf direction.
Various designs are employed to resolve the leakage rate, including stepped edges to the leaves, which therefore interlock (to an extent) and limit the clear view between leaves. However, inter-leaf leakage is a limiting factor in some treatment plans such as Intensity Modulated Radiation Therapy (IMRT) where the treatment time is relatively long.
The pixellation issue is a factor related to the resolution of the leaf bank, and therefore efforts to alleviate this problem tend to involve the use of narrower leaves. This does, however, make the collimator more complex and present significant engineering challenges. JP03009767 suggests the use of two banks of leaves, offset so that the leaves of one cover the gaps between leaves of the other. This results in an improved resolution and reduced leakage, but still gives a pixellated effect.